The present invention relates to a process for detecting cracks in rotating shafts.
The early detection of a crack in a shaft is particularly important in the case of large machines of the type used in power plants since this prevents damage to such machines as well as the associated costs and the risks. Since these machines are often in continuous operation for prolonged periods of time and since it is not economically feasible to temporarily take them out of operation, processes have already been developed in which, by means of continuous monitoring and analysis of the flexural vibrations in the shaft, an attempt is made to detect the presence of a crack in the shaft on the basis of certain changes in the vibration.
Thus, U.S. Pat. No. 4,380,172 discloses a process for determining cracks in a turbine rotor in which the vibrations of the rotor are picked up and evaluated during the operation of the turbine under load as well as at an essentially normal operating speed. For this purpose, there are vibration sensors positioned at 90.degree. with respect to each other along the circumference of the bearings, and these sensors can be designed as displacement or acceleration pick-ups. These vibration signals are first picked up when the turbine is in its normal operation, processed and then the harmonic components of the vibration signals are determined by means of signal analyses. As a result, the vibration components appear with single, double and triple rotational frequencies. In order to detect cracks as soon as possible on the basis of an evaluation of these vibration components, the temperature of the turbine rotor is changed, for example, by means of an appropriate regulation of the steam temperature, as a result of which thermal stresses are temporarily created in the rotor which, in turn, influence the crack-related vibration behavior of the rotor. The existence of a crack is ascertained by comparing the vibration signals picked up and analyzed before and after the temperature change, and particularly by observing the change of their two-fold rotational-frequent components. The known process, however, has proven to be insufficiently informative since unbalance, alignment errors, bearing damage and innumerable other influences can give rise to vibration phenomena which are similar to the vibration phenomena due to a crack. Field experiments have showed that the evidence of a crack can be hidden by the simultaneous occurrence of various disturbances. It was also observed that two-pole generators excite two-fold rotational-frequent vibrations in connection with rotational-frequent variations without the presence of a crack.
In order to distinguish vibration characteristics of a shaft crack from other function disturbances which result in a similar vibration behavior, it is a known process to form the kinetic shaft orbit or shaft-vibration orbit on the basis of the picked-up signals and to then represent and evaluate these signals in a polar diagram. It is also a known process to represent and evaluate the formation of vibration figures of the filtered-out vibration component with double rotational frequency. The evaluation of the vibration figures, however, calls for a number of additional analyses, as a result of which it is very complex and expensive. There are no provisions for a decomposition of the vibration figures into forward and backward whirls.